Depends exactly what you want to know,but the basics are make a selection by whatever means you prefer, hit the quick mask button and you have a temporary mask, or hit the load selection as channel button on the channels palette and you have reusable mask. You can soften the edge of the mask by featering the selection and edit it by spraying with black grey or white. Shades of grey will give a semi-transparent mask. The best way to learn is to try a few and just mess around to see what the possibilities are.